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POType check error in Haskell monad chaining
 

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  1. POType check error in Haskell monad chaining
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    <p>I'm new to Haskell and I'm just following the example on RWH. I'm having problems with the following programs on Chapter 14:</p> <pre><code>import qualified Data.Map as M type PersonName = String type PhoneNumber = String type BillingAddress = String data MobileCarrier = Honest_Bobs_Phone_Network | Morrisas_Marvelous_Mobiles | Petes_Plutocratic_Phones deriving (Eq, Ord) findCarrierBillingAddress :: PersonName -&gt; M.Map PersonName PhoneNumber -&gt; M.Map PhoneNumber MobileCarrier -&gt; M.Map MobileCarrier BillingAddress -&gt; Maybe BillingAddress -- This will work findCarrierBillingAddress person phoneMap carrierMap addressMap = do phone &lt;- M.lookup person phoneMap carrier &lt;- M.lookup phone carrierMap address &lt;- M.lookup carrier addressMap return address -- This will NOT work: findCarrierBillingAddress person phoneMap carrierMap addressMap = return person &gt;&gt;= lookup phoneMap &gt;&gt;= lookup carrierMap &gt;&gt;= lookup addressMap where lookup = flip M.lookup </code></pre> <p>It seems that when writing findCarrierBillingAddres in the monad chaining format using >>=, it just does not type check:</p> <pre><code>/home/bruce/Programming/haskell/real/ch14/hello.hs:21:9: Couldn't match type `[Char]' with `MobileCarrier' Expected type: MobileCarrier -&gt; Maybe BillingAddress Actual type: PersonName -&gt; Maybe BillingAddress In the return type of a call of `lookup' In the second argument of `(&gt;&gt;=)', namely `lookup addressMap' In the expression: return person &gt;&gt;= lookup phoneMap &gt;&gt;= lookup carrierMap &gt;&gt;= lookup addressMap /home/bruce/Programming/haskell/real/ch14/hello.hs:21:16: Couldn't match type `MobileCarrier' with `[Char]' Expected type: M.Map PersonName BillingAddress Actual type: M.Map MobileCarrier BillingAddress In the first argument of `lookup', namely `addressMap' In the second argument of `(&gt;&gt;=)', namely `lookup addressMap' In the expression: return person &gt;&gt;= lookup phoneMap &gt;&gt;= lookup carrierMap &gt;&gt;= lookup addressMap Failed, modules loaded: none. </code></pre> <p>The question is.. why the second format using >>= would not type check? </p>
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    1. COYou should make person the last argument. It seems better for partial application. And you can rewrite using Kleisli composition, <=<.
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